Problem Decrypting Data

[Patrick Burrows]

Hi,

I am a newbie to working with CryptLib and was wondering if someone could help me figure out what I am doing wrong decrypting data. I've followed the documentation very carefully and seem to be doing everything correct. But, well, obviously I am not.

This is a simple password encryption using C# [1]. Pretty much the most basic usage of the library, from what I can tell. 

The one part I am not following to the letter is that I am calling the cryptSetAttribute methods *before* I call cryptPushData.

But when I reverse those two calls, I get an error (and not the expected error telling me I need a password, rather error -21 (No Permission to Perform this Operation.))

Any help would be greatly appreciated. I've tried every variation of this code that I can imagine. The full source of the entire (short, simple) console application is here:

http://csharp.pastebin.com/m472c8e42

[1]

        static string DecryptData(string encodedData, string password)

        {

            byte[] envelopedData = StringToBytes(encodedData);

            int envelope = crypt.CreateEnvelope(crypt.UNUSED, crypt.FORMAT_CRYPTLIB);            

            crypt.SetAttribute(envelope, crypt.ENVINFO_DATASIZE, envelopedData.Length);

            crypt.SetAttributeString(envelope, crypt.ENVINFO_PASSWORD, password);

            int bytesCopied = 0;

            try

            {

                bytesCopied = crypt.PushData(envelope, envelopedData);

            }

            catch (CryptException cexc)

            {

                Console.WriteLine(cexc.ToString());

            }

            

            crypt.FlushData(envelope);

            byte[] messageBuffer = new byte[envelopedData.Length];

            bytesCopied = crypt.PopData(envelope, messageBuffer, messageBuffer.Length);

            string unencryptedData = ASCIIEncoding.ASCII.GetString(messageBuffer);

            crypt.DestroyEnvelope(envelope);

            return unencryptedData;

        }

--
Patrick Burrows


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[Wolfgang Gothier]

> The one part I am not following to the letter is that I am calling the
> cryptSetAttribute methods *before* I call cryptPushData.
>
> But when I reverse those two calls, I get an error (and not the expected
> error telling me I need a password, rather error -21 (No Permission to
> Perform this Operation.))
>
> Any help would be greatly appreciated.

When calling crypt.CreateEnvelope(crypt.UNUSED,crypt.FORMAT_CRYPTLIB)
you try to encrypt and NOT to decrypt the message! That's why you get an
error -21 when you reverse your callind sequence.
You have to use crypt.FORMAT_AUTO as second parameter for decryption.

Using envelopedData.Length as length of your messageBuffer is not
the optimal solution: In case of an encrypted envelope the decrypted
length is always shorter than the envelope length, but when you try
to de-envelope a compressed envelope, you may get a longer byte
string on PopData than you pushed in.

Wolfgang Gothier


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